An air dryer for removing moisture and oil in the compressed air output from an air compressor in a brake system for a large-size automobile, for example, is known. Some air dryers incorporate a pressure governor, which outputs a control order to an air compressor outside the air dryer or to a drain valve in the air dryer based on the air pressure received from an air tank for storing dry air and maintains the pressure in the air tank within a prescribed range (refer to Patent Literature 1, for example).